Failing that, you could try record by using the built-in audio (Realtek?) in your computer.
It would involve connecting a microphone headset to the PC jack.
Then set up Sonar to record from the internal audio hardware.
 
If that works, it's beginning to point at the interface (or driver)

 I just tried a 3rd audio source, live microphone recording.  The problem still persists, but I also tried recording using Sound Forge and again, the problem is there.  So I can rule out Sonar. 
 
Here's a possibility:  We bought an electric car last week and had a 240volt charger installed in the garage.  But it's on different circuit from my studio and it's at least 20 or 30 feet away from my studio.   I hope this isn't the issue!   I cannot think of anything that has changed other than that.  Older wave files play back fine, MIDI plays back fine.  Only when I record something new does the static appear.  And one other thing, if there is no sound source at the time of recording, there is no crackling.  The crackling seems coupled to the audio signal.  Hmm....
 

I shouldn't think it's the charger.
That would cause spikes in the audio, and they would be more regular.
 
What is clearly happening is the waveform dives to zero for a few samples.
So if there is no audio coming in, the line is already always at zero which means there's going to be no crackling.
 
Looks like a fault of some kind.
But where...
 

Are you using a power conditioner?  

When you say 3rd audio source, I assume all sources so far have had to pass through the interface?

OH, I beg your pardon, you already stated:
 
"I've tried recording from a soft synth running under Sonar and recording a MIDI track from my external VSL library computer, but it doesn't seem to matter the source, the problem is still heard."
 
OK, so it's also happening when you record without the interface.
What I'd try is do a software recording with something else, like Audacity recording a 1kHz "chirp" (that's a feature in Audacity where you can create a short clip of a tone)

jpetersen
I shouldn't think it's the charger.
That would cause spikes in the audio, and they would be more regular.
 
What is clearly happening is the waveform dives to zero for a few samples.
So if there is no audio coming in, the line is already always at zero which means there's going to be no crackling.
 
Looks like a fault of some kind.
But where...
 

No, it's not the charger.  I turned it off at the circuit breaker and the problem still persists.  It may be 1248.  I swapped USB cables, so it's not that either.  urgh!!  troubleshooting all day and I wanted to compose.  ;>(
 
 
PROBLEM FOUND!!!!!   Hallelujah!!! 
 
MOTU screwed up, they just released a new audio driver for the 1248 as well as new firmware.  The firmware won't even install, something wrong there, but I did install the new driver.  The newest drivers are always the best, right?  Wrong.  This was the culprit.  I rolled back to the earlier driver and the static is gone.  
 
Persistence is the key to successful troubleshooting.  You really have to try everything....Too bad we don't know the order to start the process of elimination before beginning the troubleshooting!
 
Thanks again for caring enough to respond!
Jerry
www.jerrygerber.com
 
 
 

Getting late, I have to log out.
I have no more ideas anyway. But I'm interested in the problem and will look in tomorrow morning.
 
But just to sum up: it is not interference, not from phones nor chargers.
The audio waveform strongly suggests some hardware or software component cannot keep up 
and drops out of operation for some samples.
 
Edit: Just saw your latest post. Excellent! Now I can sleep peacefully.

I record on the same circuit as a chevy volt that wall charges. I've never had a related issue.

Enjoy your electric car!

gswitz
I record on the same circuit as a chevy volt that wall charges. I've never had a related issue.

Enjoy your electric car!

Thanks, I just bought the Chevy Bolt.  Wow!  What a car, 238 miles per charge and no gasoline.   Chevy really did their homework on this one...